This invention relates to a pneumatic suspension system for a motor vehicle including a compressor unit, a storage unit, as well as on-off valves via which individual pneumatic springs of the individual vehicle wheels are supplied with compressed air. In the area between the compressor unit and the on-off valves, a vent line is provided that can be locked or that can be released. The vent line opens up into the atmosphere. As for the technical field, reference is merely made by way of example to German patent document DE 29 42 081 C2.
Pneumatic suspension systems are distinguished from all mechanically buffered wheel suspensions by greater flexibility or greater variability. In the case of two-axle pneumatic systems, for example, the height of the vehicle superstructure with respect to the ground and, thus, the ground clearance of the motor vehicle, can be altered. Usually, one employs so-called open pneumatic systems here where (in two-axle pneumatic systems), when the vehicle superstructure is lowered from a raised position, the air mass that is to be evacuated for this purpose from the pneumatic springs is evacuated by a so-called vent line and out into the atmosphere, that is to say, into the environment. If this lowering action is to take place within a relatively short time span, then the cross-sectionxe2x80x94more specifically, the clear through flow profile of the vent line that is usually provided in the vicinity of the compressor unit of the pneumatic system and that can be closed-off or cleared by means of a locking valvexe2x80x94must have a certain minimum size.
In this connection, reference is made expressly to the fact that the invention at hand is not confined to two-axle pneumatic systems, but can also be used equally for single-axle pneumatic systems. It might furthermore be mentioned that the static air pressurexe2x80x94that is located upstream from the still closed on-off valves and that thus is applied in the pneumatic springsxe2x80x94can be within the order of magnitude of 4 to 10 bar; the dynamic pressure values can be definitely higher. The total system pressure also is usually higher, that is to say, up to the level of the pressure in a storage unit.
Along with a compressor unit (with whose help the system pressure, required for charging the mentioned pneumatic springs, or the pertinent required air mass that will be supplied in the final analysis), there is also usually provided an already briefly mentioned storage unit. Placed in the storage unit for intermediate storage are air masses that are compressed by the compressor unit, since a reasonably dimensioned compressor would not be able to supply the required air mass for the desired lifting of the vehicle superstructure, within a practical amount of time. If the vehicle superstructure is then to be raised, one can draw the necessary air mass, which is subjected to adequate system pressure, from the preferably completely filled storage unit.
This explanation clearly shows that the pressure, prevailing in the filled storage unit, must be greater than the pressure usually applied in the pneumatic springs. This is because, otherwise, it would not be possible to fill those springs any further. By way of example, the pressure, prevailing in the filled storage unit, can be on the order of 15 bar. It is furthermore obvious that the compressor unit can be shut down after the complete filling of the storage unit, so long as no air mass is drawn from the storage unit. In this stage, it is desirable that one reduce the pressure applied in the pertinent line system, that is to say, in the line system between the compressor unit as well as the mentioned on-off valves, which, for instance, can be combined into a so-called valve block and on whose sidexe2x80x94facing away from the compressor unitxe2x80x94the individual pneumatic springs are connected. In other words, one can aerate or vent this line system. A venting of this line system, that is to say, of the compressor unit, as well as the line segment between the compressor unit and the valve block or the on-off valves (naturally, without the storage unit that branches off from that line section and that can be closed off), is basically possible here via the previously mentioned venting line that is provided in the area between the compressor unit and the valve block.
But it was found that this aeration, venting or pressure release of the system (which initially is under the (higher) pressure of 15 bar, mentioned earlier by way of example) via this venting line is accompanied by an undesirable, practically no longer acceptable noise generation. In case of a desired lowering of the vehicle superstructure, starting with a pressure level of 4-10 bar, for instance, only part of the air mass contained in the system is evacuated in an abrupt manner via this venting line, whose pressure level is reduced compared to the higher pressure level of, for instance, 15 bar. Of course, a sound absorber or the like could be provided in the vent line. However, such an absorber represents a choke that works in an undesirable manner and that is always present, and, therefore, also in case the vehicle superstructure is lowered.
The object of the present invention is to point out a comparatively more favorable expedient in helping to solve the complex problem just described. The solution of this problem is characterized by measures with whose help one can limit the air mass flow that emerges via the cleared vent line during a certain time interval, as compared to an unlimited state. Advantageous embodiments and developments are covered in the subclaims and described herein.
According to the invention, the mass of the airflow that is conducted through the vent line can be limited. Specifically, when the pressure in the system is to be reduced between the compressor unit and the valve block or the on-off valves, that is to say, after the storage unit has been completely filled, the mass of air flow is limited. So that this, previously described, relatively high pressure will not be reduced abruptly (thus generating noises) via the vent line, it is possible for these cases to limit the magnitude of the air mass that is moved through the vent line within a certain time interval, that is to say, to limit the air mass flow. On the other hand, if the vehicle superstructure is lowered and if, accordingly, a part of the air mass, located in the pneumatic springs, is also evacuated via this vent line into the atmosphere, there should not be any such restriction or limitation, that is to say, in that case, the full flow cross section of the vent line should be available to facilitate the relatively fast lowering of the vehicle superstructure.
Basically, there are various possibilities of an optional restriction or a release, that is to say, lifting of this restriction or limitation of the air mass flow that can be conducted through the vent line within a certain time span. By way of example, using a switchable branch, one can optionally connect a sound absorber into the vent line, although this solution requires a relatively large amount of structural space.
Comparatively more favorable measures are listed in the subclaims and will be described in greater detail in the following.